PRINCE LOUIS finally made his debut in the Cambridge family Christmas card photos today, as Kensington Palace released a beautiful picture of Kate Middleton and Prince William with all three of their children. But why is the tot wearing tights?

Kate Middleton, 36, and Prince William, 36, delighted royal fans today as Kensington Palace shared the first official family photo of the Duke and Duchess of Cambridge with their three children, Prince George, five, Princess Charlotte, three, and Prince Louis, eight months. Prince Louis looks adorable in the picture, smiling at the camera as his mother Catherine holds him in her arms while they all perch on a large branch of a tree in the gardens of Anmer Hall. While Prince George looks grown up in a pair of trousers and a khaki knitted jumper, his baby brother is dressed in a light blue jumper and a pair of navy shorts with tights beneath. Louis also had little blue booties on his feet in the heartwarming snap.

When Prince George was a toddler, he was often pictured wearing shorts when accompanying his parents to royal engagements and this often sparked confusion amongst parents and royal fans.

But the style is all down to tradition, as it’s often considered “suburban” not to do so.

As the family’s Christmas card photo was taken in the autumn, baby Louis would have been dressed in tights to keep him warm so that he could still wear the traditional shorts.

Etiquette expert William Hanson told Harpers Bazaar: “It's a very English thing to dress a young boy in shorts.

"Trousers are for older boys and men, whereas shorts on young boys is one of those silent class markers that we have in England.

“Although times are (slowly) changing, a pair of trousers on a young boy is considered quite middle class – quite suburban. And no self-respecting aristo or royal would want to be considered suburban. Even the Duchess of Cambridge."

The Cambridges' Christmas card photo (Image: PA)

Prince Louis looked adorable in the family photo (Image: PA)

In old pictures of Princes William and Harry when they were growing up, the boys would often wear shorts.

"The usual custom is that a boy graduates to trousers around eight years old," the etiquette expert explained.

"This is, historically, perhaps due to the practice of 'breeching', which dates back to the sixteenth century. A newborn boy would be dressed in a gown for their first year or two (these gowns have survived as the modern Christening robe) and then he was 'breeched' and wore articles of clothing that more resembled shorts or trousers than dresses."

Prince George is five years old but was dressed in trousers for the portrait, after years of wearing shorts as a tot.

Meanwhile, Princess Charlotte looks adorable stood between her parents and her brothers, with one arm on her mother and the other around her big brother George.

The three year old also wore her hair in a ponytail and a denim dress beneath a blue knit jumper.